A facile approach to direct growth of layer-tunable graphene on Ge substrates

化学气相沉积法(CVD)在制备高质量、大尺寸石墨烯方面显示出巨大 的优势 [11~13] , 该方法成本低廉, 更能够和目前的集 成 电 路 工 艺 相 兼 容 .

Many applications of graphene especially for the fabrication of electrical devices require physical placement of graphene on dielectric substrates. Several strategies developed for the graphene device fabrication include 1) mechanical exfoliation of graphene on a dielectric substrate followed by fabrication, 2) graphene growth on metal catalysts by chemical vapor deposition (CVD) process followed by transferring it onto dielectric substrates and fabrication, 3) dispersion of reduced graphene oxides (RGOs) on dielectric substrates followed by fabrication, etc. To utilize the intrinsic high charge carrier mobility of graphene, however, a direct synthesis of graphene by CVD on dielectric substrates is highly demanding. In this presentation, I will introduce our recent experimental results demonstrating successful direct growth of graphene on various dielectric substrates, such as hexagonal boron nitride (h-BN), sapphire, quartz, and amorphous SiO2/Si, thus excluding complex transfer process to secure the quality of graphene by excluding impurities that are accompanied during the transfer process. The detailed growth mechanism involved in graphene nucleation and growth propagation on various substrates will be also discussed.

Direct synthesis of graphene with well-defined nanoscale pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom-up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, it is shown that a simple reduction in temperature during chemical vapor deposition (CVD) on Cu induces in-situ formation of nanoscale defects (≤2-3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution-casting of hierarchically porous polyether sulfone supports on the as-grown nanoporous CVD graphene, large-area (>5 cm2 ) NATMs for dialysis applications are demonstrated. The synthesized NATMs show size-selective diffusive transport and effective separation of small molecules and salts from a model protein, with ≈2-100× increase in permeance along with selectivity better than or comparable to state-of-the-art commercially available polymeric dialysis membranes. The membranes constitute the largest fully functional NATMs fabricated via bottom-up nanopore formation, and can be easily scaled up to larger sizes permitted by CVD synthesis. The results highlight synergistic benefits in blending traditional membrane casting with bottom-up pore creation during graphene CVD for advancing NATMs toward practical applications.

Direct synthesis of graphene nanoribbons on dielectric or semiconducting substrates offers a scalable route for integration of graphene-based devices into a conventional silicon-based technology.(1) So-far, wafer-scale synthesis of armchair graphene nanoribbons has been demonstrated on Ge (001) using chemical vapor deposition.(2) However, direct synthesis of graphene or graphene nanoribbons has not yet been achieved on CMOS compatible Si(001) due to formation of stable SiC at temperatures > 1000 K, which are needed to achieve synthesis of nanoribbons with smooth, armchair edges. A promising and realistic way to overcome this challenge is to synthesize graphene nanoribbons on CMOS compatible Ge(001)/Si(001) substrates. So far the synthesis of monolayer graphene on Ge(001)/Si(001) substrates has been demonstrated by CVD.(3, 4) In this study, we synthesized graphene nanoribbons on 3 µm epitaxial Ge(001)/Si(001) substrates, by ambient pressure CVD using methane as the carbon precursor. We show that growth kinetics of graphene nanoribbons on Ge(001)/Si(001) are comparable to that on Ge(001). By tuning the methane flow and growth time we are able to synthesize graphene nanoribbons ranging from 100 nm to 1 µm in length with high aspect ratios, whilst avoiding Si diffusion from the bulk. For instance, by restricting methane to < 6000 ppm and growth time to < 3h, graphene nanoribbons with widths < 10 nm can be synthesized with aspect ratios as high as 70. Such nanoribbons grown on Ge(001) have been shown to exhibit technologically relevant band-gaps as well as exceptional charge transport properties.(5) Furthermore, we also investigated the possible role of threading dislocations in Ge epilayer on nucleation or growth of nanoribbons and show that nanoribbons readily grow over the threading dislocations. Lastly, we studied the evolution of surface roughness with the nucleation density of nanoribbons. Our study provides valuable insight into the mechanism of graphene nanoribbon growth on Ge(001)/Si(001) substrates. From this information, it is expected that unidirectional graphene nanoribbons with rational placement and control over width poly-dispersity can be synthesized on Ge(001)/Si(001) platform akin to Ge(001), which has been demonstrated recently using seed-mediated growth.(6) This provides a scalable way for wafer scale integration of graphene nanoribbon arrays on Si and provides motivation for further research into this direction. References A. Khan et al., Direct CVD Growth of Graphene on Technologically Important Dielectric and Semiconducting Substrates. Advanced Science 5, (2018).R. M. Jacobberger et al., Direct oriented growth of armchair graphene nanoribbons on germanium. Nature Communications 6, (2015).I. Pasternak et al., Graphene growth on Ge(100)/Si(100) substrates by CVD method. Scientific Reports 6, (2016).M. Lukosius et al., Metal-Free CVD Graphene Synthesis on 200 mm Ge/Si(001) Substrates. Acs Applied Materials & Interfaces 8, 33786-33793 (2016).R. M. Jacobberger, M. S. Arnold, High-Performance Charge Transport in Semiconducting Armchair Graphene Nanoribbons Grown Directly on Germanium. Acs Nano 11, 8924-8929 (2017).A. J. Way, R. M. Jacobberger, M. S. Arnold, Seed-Initiated Anisotropic Growth of Unidirectional Armchair Graphene Nanoribbon Arrays on Germanium. Nano Letters 18, 898-906 (2018). Figure 1

The synthesis of transfer-free graphene is necessary for expanding its industrial applications. Although the direct synthesis of graphene on the insulating substrate via a metal sacrificial film was reported, the growth mechanism of transfer-free graphene still remains to be studied. Herein, a detailed synthesis model of graphene grown from different carbon sources has been established to help in selecting the growth conditions for high-quality graphene. A detailed discussion on the critical influence of dissolution and the diffusion rate of carbon atoms on the growth process has also been presented. The high decomposition temperature carbon sources promote the formation of high-quality monolayers of graphene. The carbon diffusion rate of the Cu film is significantly higher than that of Ni. This promotes the synthesis of graphene from methane and diamond-like carbon. However, adverse effects are exerted on polymethyl methacrylate. Ion implantation technology and different components of the Ni–Cu alloy were used to understand this growth mechanism. This work could guide the growth conditions of transfer-free, large-scale, and high-quality graphene that can be potentially used for the fabrication of a semiconductor or an insulation substrate in theory. The reported method can generate interest in the field and increase the industrial applications of graphene-based devices that exhibit rough or patterned surfaces.

The direct synthesis of graphene with high-quality on semiconducting germanium (Ge) substrates has been developed recently, which has provided a promising way to integrate graphene with semiconductors for the application of electronic devices. However, the defects such as grain boundaries (GBs) introduced during the growth process have a significant influence on the crystalline quality of graphene and the performance of related electronic devices. Therefore, the investigation of the formation of GBs in graphene grown on a Ge substrate is essential for optimizing the crystalline quality of graphene. Herein, the formation mechanism and microstructure of GBs in graphene grown on Ge (110), Ge (001), and Ge (111) substrates via a chemical vapor deposition method are revealed. Ex situ atomic force microscopy is utilized to monitor the evolution of graphene domains. It is found that a single crystalline graphene film without GBs is formed on Ge (110), while polycrystalline graphene films with GBs are grown on Ge (001) and Ge (111) substrates, as suggested by transmission electron microscopy and x-ray photoelectron spectroscopy measurements. Our work may motivate the future exploration in improving the crystalline quality of graphene grown on a semiconducting substrate and the performance of associated electronic devices.

Integration of graphene with Si-based semiconductor materials, which are core-materials in complementary metal oxide semiconductor (CMOS) technology, is desirable. To date, the synergistic effects of various graphene-semiconductor hybrid systems have been shown to overcome various physical limitations of graphene and semiconductors. However, the graphene utilized in most previous studies was synthesized over a metal catalyst and required additional transfer processes that could generate irreversible physical defects and chemical contamination. Although direct synthesis of graphene on Si or Ge has been developed, research into graphene growth using silicon-germanium (SiGe) as a catalyst remains in its infancy. This is despite expectations that this method would be highly applicable to next-generation CMOS applications. Herein, we demonstrate the direct growth of graphene on a SiGe surface using a conventional chemical vapor deposition method. Optical microscopy, electron microscopy, and Raman spectroscopy were used to confirmed that the graphene was uniformly synthesized over the entire substrate. The advantages of CMOS-compatible graphene growth and high crystallinity of the synthesized graphene will provide opportunities for novel graphene-SiGe hybrid system development and next-generation CMOS technology.

Deposition of layers of graphene on silicon has the potential for a wide range of optoelectronic and mechanical applications. However, direct growth of graphene on silicon has been difficult due to the inert, oxidized silicon surfaces. Transferring graphene from metallic growth substrates to silicon is not a good solution either, because most transfer methods involve multiple steps that often lead to polymer residues or degradation of sample quality. Here we report a single-step method for large-area direct growth of continuous horizontal graphene sheets and vertical graphene nano-walls on silicon substrates by plasma-enhanced chemical vapor deposition (PECVD) without active heating. Comprehensive studies utilizing Raman spectroscopy, x-ray/ultraviolet photoelectron spectroscopy (XPS/UPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and optical transmission are carried out to characterize the quality and properties of these samples. Data gathered by the residual gas analyzer (RGA) during the growth process further provide information about the synthesis mechanism. Additionally, ultra-low friction (with a frictional coefficient ∼0.015) on multilayer graphene-covered silicon surface is achieved, which is approaching the superlubricity limit (for frictional coefficients <0.01). Our growth method therefore opens up a new pathway towards scalable and direct integration of graphene into silicon technology for potential applications ranging from structural superlubricity to nanoelectronics, optoelectronics, and even the next-generation lithium-ion batteries.

Chemical vapor deposition (CVD) on catalytic metal surfaces is considered to be the most effective way to obtain large-area, high-quality graphene films. For practical applications, a transfer process from metal catalysts to target substrates (e.g., poly(ethylene terephthalate) (PET), glass, and SiO2 /Si) is unavoidable and severely degrades the quality of graphene. In particular, the direct growth of graphene on glass can avoid the tedious transfer process and endow traditional glass with prominent electrical and thermal conductivities. Such a combination of graphene and glass creates a new type of glass, the so-called "super graphene glass," which has attracted great interest from the viewpoints of both fundamental research and daily-life applications. In the last few years, great progress has been achieved in pursuit of this goal. Here, these growth methods as well as the specific growth mechanisms of graphene on glass surfaces are summarized. The typical techniques developed include direct thermal CVD growth, molten-bed CVD growth, metal-catalyst-assisted growth, and plasma-enhanced growth. Emphasis is placed on the strategy of growth corresponding to the different natures of glass substrates. A comprehensive understanding of graphene growth on nonmetal glass substrates and the latest status of "super graphene glass" production are provided.

Utilizing a direct chemical vapor deposition (CVD) approach to allow transfer‐free synthesis of graphene on insulating substrates is appealing. Nevertheless, the quality and uniformity of thus‐grown graphene without the aid of any catalyst stay normally inferior to that of graphene formed on metals by far. This mainly stems from the catalytic inertness of the insulating surface with regard to the decomposition of carbon feedstock. In this respect, delicate methodologies have been devised to date to boost the quality of directly‐grown graphene. In this Minireview, recent advances in the direct CVD growth of graphene on insulators by introducing various gaseous promotors is covered. The effects and mechanisms of different types of promotors on the direct growth are discussed. At the end, existing challenges and future perspectives in this field are further highlighted.

A systematic study of the growth and characterization of few-layer graphene on Ni foils

As one of the most appealing materials, graphene possesses remarkable electric, thermal, photoelectric and mechanic characteristics, which make it extremely valuable both for fundamental researches and practical applications. Nowadays the synthesis of graphene is commonly achieved by growing on metal substrate via chemical vapor deposition. For the integration in micro-electric device, the as-grown graphene needs to be transferred onto target dielectric layer. However, wrinkles, cracks, damages, and chemical residues from the metal substrate and the auxiliary polymer are inevitably introduced to graphene during such a transfer process, which are greatly detrimental to the performances of the graphene devices. Therefore, the direct synthesis of graphene on dielectric layer is of great importance. Many researches about this subject have been carried out in the last few years. While only few papers have systematically reviewed the direct growth of graphene on dielectric layer. For the in-depth understanding and further research of it, a detailed overview is required. In this paper, we summarize the recent research progress of the direct syntheses of graphene on dielectric layers, and expatiate upon different growth methods, including metal assisted growth, plasma enhanced growth, thermodynamics versus kinetics tailored growth, et al. Then differences in property between graphenes grown on various dielectric and insulating layers which serve as growth substrates in the direct growing process are discussed, such as SiO2/Si, Al2O3, SrTiO3, h-BN, SiC, Si3N4 and glass. Some kinds of mechanisms for graphene to be directly grown on dielectric layers have been proposed in different reports. Here in this paper, we review the possible growth mechanisms and divide them into van der Waals epitaxial growth and catalytic growth by SiC nanoparticles or oxygen atoms. Detailed data including Raman signals, sheet resistances, transmittances, carrier motilities are listed for the direct comparison of the quality among the graphenes grown on dielectric layers. The research focus and major problems existing in this field are presented in the last part of this paper. We also prospect the possible developing trend in the direct syntheses of high quality graphenes on dielectric layers in the future.

Direct synthesis of high-quality graphene on dielectric substrates is of great importance for the application of graphene-based electronics and optoelectronics. However, high-quality and uniform graphene film growth on dielectric substrates has proven challenging due to limited catalytic ability of dielectric substrates. Here, by employing a Cu ion implantation assisted method, high-quality and uniform graphene can be directly formed on various dielectric substrates including SiO2/Si, quartz glass, and sapphire substrates. The growth rate of graphene on the dielectric substrates was significantly improved due to the catalysis of Cu. Moreover, during the graphene growth process, the Cu atoms gradually evaporated away without involving any metal contamination. Furthermore, an interesting growth behavior of graphene on sapphire substrate was observed, and the results show the graphene domains growth tends to grow along the sapphire flat terraces. The ion implantation assisted approach could open up a new pathway for the direct synthesis of graphene and promote the potential application of graphene in electronics.

We present a novel method for the direct metal-free growth of graphene on quartz substrate. The direct-grown graphene yields excellent nonlinear saturable absorption properties and is demonstrated to be suitable as a saturable absorber (SA) for an ultrafast solid-state laser. Nearly Fourier-limited 367 fs was obtained at a central wavelength of 1048 nm with a repetition rate of 105.7 MHz. At a pump power of 7.95 W, the average output power was 1.93 W and the highest pulse energy reached 18.3 nJ, with a peak power of 49.8 kW. Our work opens an easy route for making a reliable graphene SA with a mode-locking technique and also displays an exciting prospect in making low-cost and ultrafast lasers.

A novel method for direct growth of a few-layer graphene on Al2O3 film